The interest in deploying low-power nodes (such as pico base stations, home eNodeBs, relays, remote radio heads, etc.) for enhancing the macro network performance in terms of the network coverage, network capacity, and service experience of individual users has been constantly increasing over the last few years. At the same time, there is a need for enhanced interference management techniques to address the arising interference issues caused, for example, by a significant transmit power variation among different cells and cell association techniques developed earlier for more uniform networks.
In 3 GPP, heterogeneous network deployments have been defined as deployments where low-power nodes of different transmit powers are placed throughout a macro-cell layout, implying also non-uniform traffic distribution. Such deployments are, for example, effective for capacity extension in certain areas, so-called traffic hotspots, i.e., small geographical areas with a higher user density and/or higher traffic intensity where installation of pico nodes may be considered to enhance performance. Heterogeneous deployments may also be viewed as a way of densifying networks to adapt to traffic needs and the environment. However, heterogeneous deployments also bring challenges for which the network should be prepared to ensure efficient network operation and superior user experience.
Currently, there are three LTE base station (BS) power classes specified: wide area BS, local area BS and home BS. But additional BS classes, such as medium range BS, are expected to be introduced in the near future. The base station classes differ in that they have different levels of maximum output power and associated minimum coupling loss. This in turn defines the maximum and a typical coverage area or the size of a cell which may be efficiently served by a particular type of base station. For example, a wide area BS is typically deployed to serve a macro cell or a wide area. Hence, a wide area. BS is interchangeably termed a macro BS. On the other hand, a local area BS is typically deployed to serve a pico cell or a local area. Thus, a local area BS is interchangeably termed a pico BS. A medium range BS is typically deployed to serve a micro cell or a medium range area. So a medium range BS is interchangeably termed a micro BS. Some other requirements such as frequency error and receiver sensitivity may also differ for different BS classes as they are generally optimized for specific deployment scenarios. In LTE, the maximum output power of a local area BS, which serves a pico cell, and a home BS, which serves a femto cell, is 24 dBm and 20 dBm for a non-MIMO case, respectively. For example, in both WCDMA and E-UTRAN FDD and TDD, the home base station maximum output power is 17 dBm per antenna port in case of two transmit antennas, 14 dBm per antenna port in case of four transmit antennas, and so on.
Interference Management for Heterogeneous Deployments
To ensure reliable and high-bitrate transmissions as well as robust control channel performance, maintaining a good signal quality is a must in wireless networks. The signal quality is determined by the received signal strength and its relation to the total interference and noise received by the receiver. A good network plan including cell planning is a prerequisite for the successful network operation, but it is static. For more efficient radio resource utilization, the plan may be complemented at least by semi-static and dynamic radio resource management mechanisms, which are also intended to facilitate interference management and deploy more advanced antenna technologies and algorithms.
One way to handle interference is, for example, to adopt more advanced transceiver technologies, e.g. by implementing interference cancellation mechanisms in terminals. Another way, which may be complementary to the former, is to design efficient interference coordination algorithms and transmission schemes in the network.
Inter-cell interference coordination (ICIC) methods for coordinating data transmissions between cells have been specified in LTE release 8, where the exchange of ICIC information between cells in LTE is carried out via the X2 interface by the X2-AP protocol. Based on this information, the network may dynamically coordinate data transmissions in different cells in the time-frequency domain and also perform transmit power control so that the negative impact of inter-cell interference is minimized or at least reduced. With such coordination, base stations may optimize their resource allocation by cells either autonomously or via another network node ensuring centralized or semi-centralized resource coordination in the network. In the current 3 GPP specification, such coordination is typically transparent to UEs. Two examples of coordinating interference on data channels are illustrated in FIG. 1. In example (1), data transmissions in two cells belong to different layers, i.e. macro and pico layers, are separated in frequency. In example (2), low-interference conditions are created at some time instances for data transmissions in pico cells by suppressing macro-cell transmissions in these time instances, e.g., in order to enhance performance of UEs which would otherwise experience strong interference from macro cells, e.g., UEs closely located to macro cells.
Unlike data channels, in the current 3 GPP specification, ICIC possibilities for control channels are more limited. For example, the mechanisms illustrated in FIG. 1 are not provided for control channels or for reference signals which are measured, e.g., for mobility. Three enhanced ICIC approaches to handle the interference on control channels are illustrated in FIG. 2. Examples (1) and (3) require standardization changes, while example (2) may be implemented with the current 3 GPP standard but is limited for time division duplex (TDD) (not possible with synchronous network deployments) and is not efficient at high traffic loads. From a legacy terminal point of view, Cell-Specific Reference Signals (CRS) still need to be transmitted in all subframes, so there will still be inter-cell interference from CRS.
The idea behind interference coordination techniques, as illustrated in FIGS. 1 and 2, is that the interference from a strong interferer (e.g., a macro cell base station) is suppressed during the weaker cell's (e.g., a pico cell base station) transmissions, assuming that the weaker cell is aware of radio resources with low-interference conditions and thus may prioritize scheduling in those subframes the transmissions for users which potentially may strongly suffer from the interference caused by the strong interferers.
Although the possibilities to efficiently mitigate inter-cell interference to and from control channels are limited with the current 3 GPP standard, even less flexibility exists for dealing with interference to/from physical signals which typically have a pre-defined static resource allocation in the time-frequency space. There are three known techniques. One is signal cancellation where the channel is measured and used to restore the signal from (a limited number of) the strongest interferers. But this technique negatively impacts the receiver implementation and complexity, and channel estimation effectively limits how much of the signal energy that may be subtracted. Another is symbol-level time shifting which does not impact the standard, but is not relevant for TDD networks and networks providing an MBMS service. A third technique is complete signal muting in a subframe, e.g., not transmitting CRS in some subframes for energy efficiency reasons.
Given the limited set of techniques, all of which have drawbacks, there is a need for a simple and efficient technique to resolve the CRS interference issue. A similar issue exists for synchronization and broadcast channels.
Cell Range Expansion
The need for enhanced ICIC techniques is particularly important when the cell assignment rule diverges from a Reference Signal Received Power (RSRP)-based approach towards a pathloss-based or pathgain-based approach. This is sometimes also referred to as cell range expansion when adopted for cells with a transmit power lower than neighbor cells. The idea of the cell range expansion is illustrated in FIG. 3, where the cell range expansion of a pico cell is implemented using a delta-parameter added to RSRP.
UE Awareness about eICIC-Related Cell Configuration
Different interference coordination techniques, also referred to as enhanced ICIC (eICIC), may be used in heterogeneous network deployments. Furthermore, to ensure robust performance for data and/or control channels and to ensure consistent UE measurements, e.g. mobility measurements, positioning measurements, channel estimation measurement, etc., in the presence of time-frequency radio resources with different interference conditions, the UE typically needs information to determine which radio resources may/should be used for those measurements that also keep the UE performance at an acceptable level. It is important to provide the UE such information and an appropriate way to provide it.
Neighbor Cell Lists in LTE
Neighbor cell lists (NCLs) are used for mobility purposes. Transmitting neighbor cell lists from the E-UTRA radio network to the UE is a required feature in 3 GPP TS 36.331, and it is optional in LTE in that the UE must meet measurement requirements (e.g., for cell search, RSRP and RSRQ accuracy) without receiving an explicit neighbor cell list from the eNode B. A similar functionality (signaling of NCL) is required in UTRA where the UE must meet more stringent measurement requirements (e.g., cell search, CPICH RSCP and CPICH Ec/No accuracy) only when an explicit neighbor cell list is signaled by the radio network controller (RNC).
In E-UTRAN, the neighbor cell information in E-UTRA may be signaled over RRC either on the Broadcast Control Channel (BCCH) logical channel in a system information block or on the Dedicated Control Channel (DCCH) in an RRC measurement configuration or reconfiguration message.
When signaled on BCCH, the neighbor cell related information for intra-frequency cell re-selection is signaled in the Information Element (IE) SystemInformationBlockType4, and the IE SystemInformationBlockType5 is used for inter-frequency cell re-selection. Both system information blocks (SIBs) are signaled over RRC dedicated signaling in the System Information (SI) message through the BCCH logical channel using RLC transparent-mode service. This SI with neighbor cell information may be acquired both in RRC_IDLE and RRC-CONNECTED states. Mapping of SIBs to SI messages is configurable by a schedulingInfoList with the restrictions that each SIB is contained in a single SI message and only SIBs having the same scheduling requirement (periodicity) may be mapped to the same SI message. The transmit periodicity of SIB4 and SIB5 may be configured as one of: 8, 16, 32, 64, 128, 256 and 512 radio frames.
The neighbor cell related information for intra-frequency cell re-selection is signaled in the IE SystemInfonnationBlockType4 includes cells with specific re-selection parameters as well as blacklisted cells. The maximum number of cells in intra-frequency NCLs or black cell list (BCL) is 16 cells. An NCL contains the Physical Cell Identities (PCIs) and corresponding cell offsets which are used to indicate a cell-specific or frequency-specific offset to be applied when evaluating candidates for cell re-selection or when evaluating triggering conditions for measurement reporting. A Black Cell List (BCL) contains a range of physical cell identities including the starting (lowest) cell identity in the range and the number of identities in the range. The Physical Cell Identity range is specified in 3 GPP TS 36.331 as follows:
PhysCellIdRange ::=SEQUENCE {startPhysCellId,rangeENUMERATED {n4, n8, n12, n16, n24, n32, n48,n64, n84,n96, n128, n168, n252, n504,spare2,spare1}OPTIONAL --Need OP}
The neighbor cell related information for inter-frequency cell re-selection signaled in the IE SystemInfonnationBlockType5 includes cell re-selection parameters common for a frequency as well as cell specific re-selection parameters. With the current 3 GPP specification, the parameters signalled per carrier frequency and optionally per cell include: carrier frequency (or ARFCN), an indicator for the presence of antenna port 1, allowed measured bandwidth, reselection parameters accounting for RSRP, an indicator for the required minimum received RSRP in the E-UTRAN cell, a reselection timer value for E-UTRA indicating the time during which the cell has to be evaluated and ranked, reselection thresholds for RSRP when reselecting toward a higher and a lower priority, and neighbor cell configuration—a bit string of two bits, used to provide the information related to MBSFN and TDD UL/DL configuration of neighbor cells (00—not all neighbor cells have the same MBSFN subframe allocation as serving cell, 10—the MBSFN subframe allocations of all neighbour cells are identical to or subsets of that in the serving cell, 01-no MBSFN subframes are present in all neighbour cells, and 11-different UL/DL allocation in neighbouring cells for TDD compared to the serving cell, for TDD, 00, 10 and 01 are only used for same UL/DL allocation in neighbouring cells compared to the serving cell).
The optional parameters that may be signalled with the current 3 GPP specification for inter-frequency NCL, per carrier frequency or per cell, include: offset (0 dB default), maximum UE transmit power (if absent the UE applies the maximum power according to the UE capability), speed-dependent scaling factor for the E-UTRA reselection timer value, absolute cell reselection priority of the concerned carrier frequency/set of frequencies, reselection thresholds for RSRP when reselecting towards a higher and a lower priority, and inter-frequency BCL.
As specified in 3 GPP TS 36.331, no UE requirements related to the contents of SystemInformationBlock4 or SystemInformationBlock5, which carry intra- and inter-frequency NCI, respectively, apply other than those specified elsewhere, e.g., within procedures using the concerned system information, and/or within the corresponding field descriptions. So in E-UTRA, the UE must meet the measurement requirements without having the NCL. But on the other hand, if the NCL is signaled, the UE must meet the current measurement requirements since the UE may ignore the NCL or complement it with a blind cell search.
To enhance the operation of heterogeneous networks, it would be helpful if UEs could obtain information about neighbor cells. However, known techniques do not specify when, how, or via which network nodes neighbor cell information may be acquired by UEs. Nor this information communicated among different network nodes. Another problem is that non-full duplex is not accounted for in such neighbor cell information. Unfortunately, the neighbor cell information defined by the current 3 GPP standard for the mobility purposes is not sufficient to ensure robust channel performance in heterogeneous networks and thus needs to be enhanced. Such an enhancement is particularly desirable for intra-frequency cells in co-channel heterogeneous network deployments. Furthermore, neighbor cell lists may not always be necessary—even in heterogeneous networks. So to avoid signaling overhead involved in sending NCLs, network efforts for creating NCLs, and UE efforts for receiving and processing such NCL information when it may not be required, some pre-defined rules for UE terminal behavior need to be specified. Furthermore, UE measurement requirements should also account for such rules.